1. Field of the Invention
The present invention relates to a throttle control device mounted on an internal combustion engine, and more particularly to a throttle control device which performs various controls such as acceleration slip control, automatic speed control, idling control, automatic speed restriction, and etc. by controlling an opening of a throttle valve with a driving power source such as a motor operated in response to operation of an accelerator operating mechanism, and which is able to stop the control effected by the driving power source at any time if necessary.
2. A throttle valve for use in an internal combustion engine is provided to regulate a mixture of fuel and air in a carburetor, or regulate an intake air flow in an electronic-controlled fuel injection system so as to control the output of the internal combustion engine, and is so structured to gear with an accelerator operating mechanism including an accelerator pedal.
Conventionally, the accelerator operating mechanism has been mechanically connected to the throttle valve, whereas a device for opening and closing the throttle valve, or controlling an opening of the throttle valve by driving means gearing with a driving power source such as a motor in response to operation of the accelerator pedal has been recently proposed. For example, Japanese Patent Laid-open Publication No. Sho 55-145867 discloses a device which is provided with a step motor connected to a throttle valve and which is arranged to drive the step motor in response to depression of the accelerator pedal.
For such a device, Japanese Patent Laid-open Publication No. Sho 59-153945 discloses various conventional countermeasures employed in the case where an electronic-controlled actuator falls into an incontrollable condition. For instance, a throttle shaft is disengaged from the electronic-controlled actuator by an electromagnetic clutch to return the throttle valve to its closed position by means of a return spring. In the above Publication (59-153945), the countermeasures have been proposed by reason that the conventional device has no driving means for opening and closing the throttle valve after the control effected by the actuator stops, so that it is unable to drive a vehicle to a certain place for repairs. Accordingly, there is disposed between a throttle shaft and a rotary shaft rotating in response to depression of an accelerator pedal, an electromagnetic clutch which disengages the shafts from each other when it is energized, while it engages the rotary shaft with the throttle shaft when it is deenergized, and there is provided a control circuit which detects the abnormality of the control operation of the electronic-controlled actuator and drives a relay to stop the power supply to the actuator and the electromagnetic clutch, whereby the throttle shaft is mechanically connected to the accelerator pedal through the electromagnetic clutch when the actuator falls into the incontrollable condition.
According to the above Publication (59-153945), the incontrollable condition of the electronic-controlled actuator is detected by another control circuit, and the power supply to the actuator and the electromagnetic clutch is stopped by that control circuit. After the above-described control stops, the rotary shaft mechanically connected to the accelerator pedal is engaged with the throttle shaft through the electromagnetic clutch. Further, regarding to the operation, it is described that since the motor generates no driving torque under the condition that the control effected by the actuator stops, it is less liable to interfere with the opening and closing operation of the throttle valve in response to depression of the accelerator pedal, with only slight force applied comparing with a depressing force of the accelerator pedal. That is, the accelerator pedal is maintained to be engaged with the actuator even after the transition to the direct throttle control by the accelerator pedal.
However, the electromagnetic clutch provided in the above-described conventional device is large in size and high in cost. In such an extreme situation that not only the electronic-controlled actuator is incontrollable, but also the control circuit is inoperative, the throttle valve might be driven continuously to open due to the radio wave noise or the like, for example. Further, in the case where a foreign body is jammed between a rotor and a stator of the motor to cause the locked condition, the throttle valve might be held under the opened condition. In this case, even if switch means is provided for stopping the power supply to the electromagnetic clutch so as to engage the throttle shaft with the accelerator pedal, there is no means for operating the throttle valve to be closed against the throttle shaft driven by the actuator, so that it is difficult to ensure a desired opening of the throttle valve, i.e. a desired throttle position.